The validation error message we get is: "Too many files, maximum '1' are allowed but '2' are given"

isValid is called twice on: A & B.

A quick look at Zend_Validate_File_Count and you can see that by setting a new destination after isValid is called, it causes the file count validator to add another file reference to its count when isValid is called again with a new destination. However, two files don't actually exist, we have simply changed the destination of the file that the file count validator already knows about.